Electrochemical Impedance Spectroscopy (EIS) has been in use for a number of years to test rechargeable batteries, such as lithium ion batteries. EIS is well suited for observing reactions in the kinetics of electrodes and batteries. In EIS, the impedance of battery over a range of frequencies is measured. Energy storage and dissipation properties of the battery can be revealed by inspecting the resulting frequency response curve. Impedance parameters such as ohmic resistance and charge transfer resistance can be estimated, for example, from a Nyquist plot of frequency response of the battery.
Other parameters that can be measured with use of EIS relate to the double layer effect, which is the formation of two layers of opposite polarity at the interface between electrode and electrolyte. The charge stored on one side is equal in value and opposite in sign with respect to the charge stored on the other side. If one of the two phases is a liquid, there is a minimum distance that the solvated ions can reach. This region of minimum distance is the so-called Helmholtz plane. The region external to the Helmholtz plane is called the outer Helmholtz layer. The ions can be located at distances below the plane. This region is called the inner Helmholtz layer. EIS is used to characterize the double layer. Parameters extracted from such characterization are used together with mathematical model of the phenomenon. Electrochemical insertion, intercalation and alloying are all processes involving the inner layer.
Another set of parameters measurable with use of EIS are diffusion and reaction parameters which change during battery charging, discharging and also are dependant on battery age, health condition and temperature. The commonly used experimental setups to parameterize electrochemical systems are cyclic voltammetry and galvanostatic cycling. In cyclic voltammetry, the potential difference is changed continuously with a fixed slope, called sweep rate. The sweep rate is changed in sign once a maximum or a minimum potential difference is reached. During this process, the current intensity is registered as function of the potential and, in general, the shape depends on the sweep rate. In galvanostatic cycling experiments, the current intensity is imposed and constant. The potential will be measured as function of the total charge passed through the system. In general, the shape of this curve is a function of the current intensity. The current intensity is changed in sign once a maximum or a minimum potential difference is reached. The shape of such curves is related to the reaction mechanism, transport of the reactants from the bulk of the phase to the interface, and transport of the product in the opposite direction
The electrode materials have to be stable in the battery electrolyte for the whole potential range used during the battery cycle, and vice versa. Such stability is achieved thanks to the formation of a protective layer called solid electrolyte interphase (SEI). It can be an oxidation/reduction product, in which case it consumes part of the charge of the battery, or a chemical product, formed by contacting the particles with the electrolyte. The SEI influences the kinetic behavior of the electrode, the irreversible charge consumed during cycling, and the cycle life.